1. Field of the Invention
This invention relates generally to power generation systems and, more particularly, this invention relates to electrochemical cells and methods for improving performance therein.
2. Description of Related Art
Electrochemical cells utilizing a reactive metal anode, an electrically conductive cathode, and an aqueous electrolyte are well known. Such cells are described in detail in numerous patents and publications, including Rowley U.S. Pat. No. 3,791,871 (February 12, 1974) and Galbraith U.S. Pat. No. 4,528,248 (July 9, 1985), the respective disclosures of which are incorporated herein by reference.
The cell disclosed in the Rowley patent typifies prior electrochemical cells and utilizes a metal anode which is highly reactive with water and spaced from a cathode by an electrically insulating film formed on the anode in the presence of water. The anode and cathode are in contact with an aqueous electrolyte during cell operation. In the cell of the Rowley patent, the anode comprises an elemental alkali metal such as sodium or lithium, and the electrolyte comprises an aqueous solution of sodium hydroxide or lithium hydroxide, respectively, produced by the electrochemical reaction of the anodic metal with water.
The anode of the Rowley patent is coated with a thin film of a non-reactive, partially water soluble material which is not electrically conductive. The film is porous and allows transport of aqueous electrolyte to the anode and transport of reaction products away from the anode. Preferably, the film is the natural hydrated oxide which forms on the metal surface as it is exposed to humid air. However, other suitable water soluble insulators may serve as the film.
The electrolyte of the cell disclosed in the Rowley patent is preferably a hydroxide of the alkali metal utilized as the anode since such hydroxide is naturally formed during operation of the cell and hence the cell automatically regenerates the electrolyte during operation. Thus, in the Rowley cell, water is introduced to the cell at a restricted rate and brought into direct contact with both the cathode and the anode. The water dissolves a portion of the soluble film on the anode, resulting in the production of a hydrated hydroxide of the anode material, plus heat. As the reaction proceeds, useful electrical power is produced.
The anode and the cathode are not in direct electrical contact with each other, but circuit connections are made at each electrode for drawing electrical power from the cell.
The alkali metal of the anode is highly reactive with water. This reactivity, however, decreases as the concentration of metal hydroxide in the electrolyte increases. As the metallic hydroxide concentration in the cell rises, the rate of power generation from the cell correspondingly diminishes, and passivation of the anode can occur if the electrolyte becomes saturated with the metal hydroxide. Thus, to maintain a desired level of power output from such electrochemical cells, relatively high concentrations of the reactive metal hydroxide should be avoided. Therefore, steps must be taken to maintain the reactive metal hydroxide concentration in the electrolyte at a level at which useful electrical current is produced. Optimally (at typical operating temperatures), the concentration of the metallic hydroxide in the electrolyte is maintained at about 80% of saturation for the electrolyte.
One solution to the problem of too great a concentration of the reactive metal hydroxide in the electrolyte is the continuous expulsion of a fraction of the electrolyte stream into the surrounding environment and the simultaneous injection of a similar flow rate of fresh water into the electrolyte. If the stream input and output are kept balanced and prorated by metal hydroxide production, this technique is effective. However, the technique has several disadvantages. Firstly, the motion of the inlet and outlet flow streams results in significant noise levels and the noise generated may exceed desired and/or tolerable noise limits. Secondly, the technique requires a continuous source of fresh feed water. For non-marine applications, there is no such ready source of inlet water and even if such inlet water were carried on board, its weight would, in most uses, be prohibitive. Accordingly, all such closed loop electrochemical cells require some form of "electrolyte management", i.e., the removal of the reactive metal hydroxide from the circulated electrolyte.
The use of simple acids, such as phosphoric acid, hydrogen fluoride, etc., as an electrolyte management agent for closed loop electrochemical cells, wherein the simple acid acts as a precipitant for the reactive metal hydroxide, generally suffers from the relatively great overhead weight burden imposed on the cell per gram of reactive metal hydroxide removed from the circulated electrolyte. Also, the extreme toxicity, volatility and dangerous propensities exhibited by some simple acids, such as hydrogen fluoride, make these materials unattractive as electrolyte management agents.